1. Field of the Invention
The present invention relates to a process for producing a biocompatible implant material. The biocompatible implant material produced by the inventive process is adapted for use as a prosthetic material for artificial bone in clinical fields such as orthopedic surgery, plastic surgery, brain surgery, oral surgery, and dental surgery.
2. Description of the Related Art
Conventionally, metallic material has been employed as a biocompatible implant material, but in recent years, ceramic material has become of interest and has been practically employed in consideration of biocompatibility. Particularly, it is known that calcium phosphate compounds have excellent biocompatibility, and that sintered products of these compounds can be chemically bound to bone or can substitute for bone.
Japanese Patent Publication (kokoku) No. 60-50744 discloses a process for producing sintered calcium phosphate which has excellent biocompatibility and high strength, in which powder predominantly comprising calcium phosphate (the atomic ratio of calcium to phosphorous is 1.4-1.75) is mixed with alkaline earth metal oxide-phosphoric acid frit such that the amount of the frit is 0.5-15 wt. % on the basis of the entirety of sintered calcium phosphate after firing; and the resultant mixture is fired. According to this process, a biocompatible implant material having excellent biocompatibility and high strength can be produced. When the thus-produced biocompatible implant material is implanted into the body, the material is chemically bound to osseous tissue and exhibits excellent effects, since the material has high strength and therefore is rarely broken. However, this material does not permit bone cells to enter therein, and therefore bone cells are not allowed to grow satisfactorily, requiring a prolonged period of time before binding of the material to the bone tissue.
Meanwhile, a porous implant material has lower strength than that of a dense material, but greater biocompatibility. Bone cells easily enter a porous material; particularly, a material comprising large pores having a size of some tens to some hundreds of .mu.m. Conventionally, there are processes for producing a porous material; for example, a process in which a combustible pore-forming material such as an organic substance or carbon is introduced into a raw material, and a process in which a foaming agent is incorporated into a raw material. Such processes have been employed for producing bricks, and when these processes are applied to calcium phosphate material, a porous implant material can be prepared.
Although a porous biocompatible implant material produced by such a conventional process comprises large pores having a size of approximately 100 .mu.m, inter-pore connection of the material is not satisfactory. Therefore, it is often observed that bone cells do not grow into the pores of such a material, and that the material has poor properties for the growth of bone. In addition, such a conventionally produced porous material has considerably low strength, and thus particles constituting the material may fall, or the material may break when it is touched or rubbed by the fingers. Namely, handling the material is difficult, and operation efficiency of the material is considerably poor when employed in a bone prosthesis operation.
In order to solve the above-described drawbacks, Japanese Patent Application Laid-Open (kokai) No. 7-194688 discloses a process in which raw material powder having a mean particle size of 5 .mu.m or less is formed into raw material granules having a particle size of 10-800 .mu.m; the granules are mixed with combustible particles having a size of 2-1,600 .mu.m; the mixture is compacted at a pressure of 1-100 kg/cm.sup.2 ; and the compacted product is fired. According to this process, a porous biocompatible implant material can be produced which comprises particle-linking structures in which particles of 10-800 .mu.m are linked together, and spaces through which pores of 2-800 .mu.m three-dimensionally connect with one another between the above structures. Unlike a conventional porous material, the porous biocompatible implant material has excellent inter-pore connection and comprises large pore-linkage portions. Therefore, the porous biocompatible implant material allows bone cells to enter the material very easily and thus exhibits excellent biocompatibility.